The present invention relates generally to the industrial use of microorganisms. More particularly, it concerns the production of organic acids in yeast.
Generally, an incubation medium having a low pH is favorable for the production of organic acids by yeast, as thereby the free acid is produced rather than the anionic form. However, the production of organic acids with microorganisms exerts a high stress on the cells: the culture medium is acidified, so that the microorganisms have to actively counteract the increased pH gradient across the plasma membrane. At low external pH (pHe), organic acids exert additional stress on the cells, as they diffuse through the plasma membrane and acidify the cytoplasm. This effect adds to the general stress exerted by low pH. Yeasts counteract this acidification, and tend to maintain a near neutral intracellular pH (pHi), but at some cost in viability and metabolic activity.
Given this stress, there is a limitation of productivity by using state of the art technology, as the yeast cells will eventually lose viability and metabolic activity. Therefore, there is interest in isolating more robust yeast strains, i.e., yeast strains capable of improved viability and metabolic activity at low pH. Any development that enables the isolation of more robust strains would be desirable for such production processes.